In order to reduce the weight of a steel sheet for a vehicle, a thickness of the steel sheet is required to be reduced. Meanwhile, in order to secure impact safety, a thickness of a steel sheet is required to be increased. In this regard, it is contradictory to secure both lighter weight and impact safety.
In order to solve this issue, it is necessary to increase the strength of a material while increasing formability thereof. This is known to be possible through various steel sheets for a vehicle such as a dual phase steel (DP steel), transformation induced plasticity steel (TRIP steel), complex phase steel (CP steel), and the like known as advanced high strength steel (AHSS).
Strength may be increased by increasing an amount of carbon in advanced high strength steel or adding an alloy element. However, considering practical aspects such as spot weldability, and the like, tensile strength to be achieved is limited to a level of about 1200 MPa.
Techniques according to the related art for achieving the product of tensile strength and elongation of 23,000 MPa % or more, have been variously developed.
In Patent Document 1, steel, containing Mn of 3.5 wt % to 9.0%, is used, thereby securing significantly excellent material properties. For example, the product of tensile strength and elongation is 30,000 MPa % or more. Meanwhile, a yield ratio is low as a level of 0.43 to 0.65, and a maximum yield strength is also low as a level of 720 MPa. Thus, there is a disadvantage in that it is difficult to compete with 1.5G-grade hot press forming (HPF) steel according to the related art having a yield strength of 1050 MPa after heat treatment.
In Patent Document 2, in a dual-phase steel, containing Mn of 2 wt % to 9%, and obtained by reverse transformation, thermal deformation occurs within a temperature range of 100° C. to Ac1+50° C. to refine a grain, and thus low temperature toughness is improved. As a result, there is an advantage in that yield strength is improved, but there may be a disadvantage in that warm transformation is performed at the end of a manufacturing process. Moreover, in the case of Patent Document 2, final annealing is performed in a batch annealing furnace (BAF), in which long time heat treatment is performed, so there may be a problem in which L-curvature of a final product is inferior, and shape properties are poor.
In Patent Document 3, a manufacturing method, in which continuous annealing is possible by increasing Ac1 temperature by adding Al to steel containing Mn of 3 wt % to 7%, is proposed. However, there may be a disadvantage in that it is difficult to secure continuous casting workability due to the addition of Al.
Meanwhile, in Patent Documents 4 and 5, a method for manufacturing a high strength steel sheet, in which tensile strength is 980 MPa or more, and the product of tensile strength and elongation is 24000 MPa % or more, using steel containing Mn of 3.5 wt % to 10%, is proposed. Here, coiling is performed at Ac1 transformation point or less after hot rolling, so it is inhibited that austenite is increased and annealed martensite is formed through first partitioning of Mn. Thus, there may be a disadvantage in that cold transformation properties are not effectively secured. Moreover, final annealing and intermediate annealing are only performed in two phase regions. Thus, it is determined that differences in hardness between ferrite and other phases in a final structure are significant, and thus it is also determined that degradation of yield strength of a final product may be caused. Moreover, in Patent Documents 4 and 5, there is no mention of yield strength, and bendability is only evaluated. Thus, a manufacturing method not suitable for complex press forming, but suitable for actual simple component forming, is proposed.